Method for Efficient Channel Estimation and Beamforming IN FDD System by Exploiting Uplink-Downlink Correspondence

ABSTRACT

A method for selecting at least one parameter for downlink data transmission with a mobile user equipment. The method is executable by a wireless communication base station having multiple antennas configured to communicate wirelessly with the mobile user equipment. The method receives an uplink probing signal from the mobile user equipment. The method determines a plurality of angles of arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas. The method transmits a plurality of downlink probing signals directionally toward corresponding angles of arrival in the plurality of angles of arrival. Each downlink probing signal is a virtual antenna port with respect to the mobile user equipment. The method receives channel state information. The method composes at least one of a rank indicator (RI), precoding matrix indicator (PMI), or modulating and coding scheme (MCS) for downlink data transmission to the mobile user equipment.

PRIORITY APPLICATION

This application is a continuation of U.S. patent application Ser. No. 14/109,211, filed on Dec. 17, 2013, now issued as U.S. Pat. No. 9,814,037, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/841,230, filed Jun. 28, 2013, each of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

Embodiments pertain to operations and communications performed by electronic devices in wireless networks. Some embodiments relate to for selecting at least one parameter for downlink data transmission with a mobile user equipment.

BACKGROUND

A typical wireless communication base station, such as a cellular system, can include multiple antennas. The multiple antennas can increase sensitivity to received signals along a desired direction, while decreasing sensitivity away from the desired direction. In addition, the multiple antennas can direct a transmitted signal along a desired direction. Both of these directional effects are desirable for users having user equipment, such as cellular telephones. For instance, the directionality in sending and receiving signals can improve reception for a cellular telephone user, and can reduce instances of dropped calls.

In general, it is computationally intensive to monitor a direction from a base station to a user, and to provide signals to and from the multiple antennas to take advantage of the directional effects. Accordingly, there exists a need for reducing the computational complexity of the directional effects from a multiple-antenna communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a base station having multiple antennas, a user having a user equipment, and multiple paths along which signals travel.

FIG. 2 illustrates a flow chart of an example of a method for selecting at least one parameter for downlink data transmission with the mobile user equipment.

FIG. 3 illustrates an example of a mobile client device on which the configurations and techniques described herein can be deployed.

FIG. 4 illustrates an example computer system that can be used as a computing platform for the computing or networking devices described herein.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments can incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments can be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.

In a wireless communication system, a user with user equipment communicates wirelessly with a base station. For example, in a cellular telephone system, a cellular telephone user sends wireless signals to a base station, and receives wireless signals from the base station. The transmitted and received signals often traverse more than one physical path from the UE to the base station. For instance, one path may be directly between the UE and the base station, while another may include a bounce off a building.

FIG. 1 illustrates an example of a base station (BS) 102 having multiple antennas 104, a user having a user equipment (UE) 106, and multiple paths 108, 110 between the base station 102 and the UE 106 along which signals travel. Each path 108, 110 has its own phase, its own power, its own angle-of-arrival (AoA) at the base station 102, and its own angle-of-departure (AoD) from the base station 102. The paths 108, 110 can change dynamically as the user moves. In some examples, the base station (BS) includes a relatively large number of antennas, such as 4, 8, 16, 32, and so forth. As the number of antennas increases, the precision of the signal directionality increases, and the computation complexity required to maintain that precision also increases. In some examples, the antennas 104 at the base station 102 are uniformly separated along one dimension. In some examples, for systems that use frequency-division duplexing (FDD) for uplink (UL) and downlink (DL) signals, the UL phase and DL phase on each path or subpath are independent of each other.

FIG. 2 illustrates a flow chart of an example of a method 200 for selecting at least one parameter for downlink data transmission with the mobile user equipment. The method can be executed by a wireless communication base station, such as base station 102, having multiple antennas, such as antennas 104, configured to communicate wirelessly with mobile user equipment, such as user equipment 106.

At 202, method 200 receives an uplink probing signal from the mobile user equipment. At 204, method 200 determines, from the received uplink probing signal, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas. At 206, method 200 transmits, from the multiple antennas, a plurality of downlink probing signals directionally toward corresponding angles-of-arrival in the plurality of angles-of-arrival. Each downlink probing signal can be a virtual antenna port with respect to the mobile user equipment. At 208, method 200 receives, from the mobile user equipment, channel state information. At 210, method 200 composes, in response to the received channel state information, at least one of a rank indicator (RI), preceding matrix indicator (PMI), or modulating and coding scheme (MCS) for downlink data transmission to the mobile user equipment. Method 200 is but one example of a method for selecting at least one parameter for downlink data transmission with the mobile user equipment; other methods may also be used.

There are three basic aspects to the method 200. In a first aspect, mobile user equipment (UE) sends an uplink probing/sounding signal. The base station (BS) then determines or estimates angles-of-arrivals (AoAs) of several significant paths. A frequency translation method is also provided. In a second aspect, the BS sends out probing signals towards the selected AoAs, each as a different antenna port towards the UE. The probing signal is a virtual antenna port, such as DFT beamform vector. Depending on how many such AoAs are selected, different numbers of antenna ports can be allocated to the UE. The UE does channel estimation and feeds back the channel state information or beam selection information to the BS. In a third aspect, the BS composes the best rank indicator (RI), precoding matrix indicator (PMI), and/or modulating and coding scheme (MCS) for downlink (DL) data transmission. Each of these aspects is discussed below in greater detail.

There are numerous potential advantages to using the method described herein. For instance, in contrast with a scheme in which the angles-of-arrival are not initially determined or estimated, where a relatively large search space is randomly probed, the method discussed herein in which the angles-of-arrival are determined from an uplink probing signal is much more efficient. Another potential advantage is the new channel state feedback. In the present design, the DL antenna ports are formed by DFT vectors as a virtual antenna port. This can be considered as being a subset of current LTE code book, especially case of 8-tx antenna. In the present design, the UL channel feedback can be simplified to focus on rank determination and beam selection, which can reduce or simplify calculations. Furthermore, a frequency translation algorithm suitable for dealing with different angles is provided.

The first aspect involves UL channel sounding for AoA estimation, the UE is scheduled by the BS (eNB) to send out sounding signals from one of its antennas. Upon receiving signals on the UL at the BS, the UE and/or the BS estimates the AoAs of the signals.

An example of an AoA estimation algorithm follows. By ULA assumption, if there are N paths, the received signal in frequency domain will be the following format:

$\begin{matrix} {{A:={{A_{1}\beta_{1}} + \ldots + {A_{N}\beta_{N}}}}{where}} & (1) \\ {\beta_{n} = \begin{bmatrix} 1 \\ e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{UL}}{F_{0}}\cos \; {(\alpha_{n})}} \\ \vdots \\ e^{{- j}\; 2\; {\pi\Delta}\; \frac{f_{UL}}{F_{0}}{({M - 1})}{\cos {(\alpha_{n})}}} \end{bmatrix}} & (2) \end{matrix}$

is the spatial signature of the n-th path, f_(UL) is the uplink frequency, F₀ is the carrier frequency, and Δ is the antenna distance in terms of wavelength at F₀. The received signal has the mathematical form of several DFT vectors.

By projecting the received signal towards different spatial signatures spanning the angular space, one can find the major power peaks along the directions. The projection,

[1,e^(j2πΔ) ^(UL) cos θ, . . . , e^(j2π(M−1)Δ) ^(UL) cos θ]

is considered for:

θ∈[0,π)

where Δ_(UL) is the antenna spacing in terms of UL wave length. Peak values of this function over [0, pi) yields the estimated AoAs of the major paths. This set of estimated AoAs is written as:

A_(est):={A₁,A_(2,) . . . , A_(N)}  (3)

This is but one example of a suitable AoA estimation algorithm; other suitable algorithms can also be used.

The first aspect also involves Per-Path frequency translation.

The uplink probing signal and the downlink probing signals can be at different frequencies. Determining the downlink probing signal, in response to the uplink probing signal, can involve determining an uplink channel vector, multiplying the uplink channel vector by a diagonal matrix to form a product, and adopting the product to be the downlink channel vector. In some examples, the diagonal matrix includes, at the Mth row and column, a complex exponential factor having an exponent that varies as (M−1) multiplied by the difference in frequency between the uplink probing signal and the downlink probing signal.

For a particular path, the UL channel and DL channel (DFT) vectors can be

$\beta_{n,{UL}} = \begin{bmatrix} 1 \\ e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{UL}}{F_{0}}{\cos {(\alpha_{n})}}} \\ \vdots \\ e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{UL}}{F_{0}}{({M - 1})}{\cos {(\alpha_{n})}}} \end{bmatrix}$ and $\beta_{n,{DL}} = \begin{bmatrix} 1 \\ e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{DL}}{F_{0}}\cos \; {(\alpha_{n})}} \\ \vdots \\ e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{DL}}{F_{0}}{({M - 1})}{\cos {(\alpha_{n})}}} \end{bmatrix}$

respectively. From the UL channel sounding/estimation process, the UL vector β_(n,UL) is determined. To get the DL vector, the following translation is applied:

${\hat{\beta}}_{n,{DL}} = {{diag}\left\{ {1,e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{DL} - f_{UL}}{F_{0}}{\cos {(a_{n})}}},\ldots \mspace{11mu},e^{{- j}\; 2\; \pi \; \Delta \; \frac{f_{DL} - f_{UL}}{F_{0}}{({M - 1})}{\cos {(a_{n})}}}} \right\} \beta_{n,{UL}}}$

This translation process is applied for each significant direction, as it is identified. In the next step this estimated DFT vector is used for DL beamforming. A further step to help is to let UE switch its transmit antennas in uplink sounding.

The first aspect also involves UE antenna switching in UL sounding for multi-rank DL transmission.

UE will switch its transmit antennas in a pre-determined manner in the UL sounding process for eNB to detect multi-rank transmission opportunities. This allows the eNB to determine if it can discriminate the antennas for supporting DL multi-stream transmission. In general, the AoA/AoD resolution increases with the number of antennas. Given a reasonable number of antennas, such as 8×8 or 16×1, and possibly different antenna spacing, more accurate information on beam directions and phase/power information can be obtained by DL proactive probing in the second aspect.

The second aspect involves DL probing with precoded beams towards significant UL AoAs. In some examples, the virtual antenna ports are specifically designed based on A_(est) from equation (3). The eNB tells the UE that N1 ports are supported in the transmission. Here N1 can take on the value of 1, 2, 4, or 8. One can choose N1 elements A_(N1) from to cover the significant directions in A_(est). Then, the eNB translates the DFT vectors in A_(N1) according to the translation equation above. Now it uses the complex transposes of them as defining (pre-coding) vector for the DL virtual CSI-RS ports. (as in Step 2.1.2 later). Note that the method works, regardless of the size of the UL/DL frequency gap, due to the per-path translation and DL CSI-RS probing.

The second aspect also involves RS transmission on top of the chosen A_(N1). The eNB applies CSI-RS signals on top of the N1 virtual ports. The resources on which the ports are transmitted are pre-determined and thus are known to the UE.

The second aspect also involves UE feedback design. The UE can measure and feedback channel state information or other measures. There are several options: As a first option, when N1 is small, e.g., less than 4, the UE uses the equations (1) and (2) above, and an older codebook for RI/CQI/PMI feedback. As a second option, when N1 is large, e.g., larger than 4, the UE first down-selects the ports into less than 4 based on CSI-RS measurement and throughput considerations, then calculates the best RI/CQI/PMI based on the selected beams. As a third option, the BS can provide explicit phase/power feedback for each port.

The third aspect involves DL data transmission. At this stage, the eNB has calculated a small set of DFT vectors suitable for data transmission towards the UE. Also, the eNB knows how to combine them with the number of data layers that are used. The eNB can use this information for data transmission, e.g., on the PDSCH channel.

Although the preceding examples of wireless network connections were provided with specific reference to 3GPP LTE/LTE-A, IEEE 802.11, and Bluetooth communication standards, it will be understood that a variety of other WWAN, WLAN, and WPAN protocols and standards can be used in connection with the techniques described herein. These standards include, but are not limited to, other standards from 3GPP (e.g., HSPA+, UMTS), IEEE 802.16 (e.g., 802.16p), or Bluetooth (e.g., Bluetooth 4.0, or like standards defined by the Bluetooth Special Interest Group) standards families. Other applicable network configurations can be included within the scope of the presently described communication networks. It will be understood that communications on such communication networks can be facilitated using any number of personal area networks, LANs, and WANs, using any combination of wired or wireless transmission mediums.

The embodiments described above can be implemented in one or a combination of hardware, firmware, and software. Various methods or techniques, or certain aspects or portions thereof, can take the form of program code (i.e., instructions) embodied in tangible media, such as flash memory, hard drives, portable storage devices, read-only memory (ROM), random-access memory (RAM), semiconductor memory devices (e.g., Electrically Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM)), magnetic disk storage media, optical storage media, and any other machine-readable storage medium or storage device wherein, when the program code is loaded into and executed by a machine, such as a computer or networking device, the machine becomes an apparatus for practicing the various techniques.

A machine-readable storage medium or other storage device can include any non-transitory mechanism for storing information in a form readable by a machine (e.g., a computer). In the case of program code executing on programmable computers, the computing device can include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. One or more programs that can implement or utilize the various techniques described herein can use an application programming interface (API), reusable controls, and the like. Such programs can be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs) can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language, and combined with hardware implementations.

FIG. 3 illustrates an example of a mobile device 300. The mobile device 300 can he a user equipment (UE), a mobile station (MS), a mobile wireless device, a mobile communication device, a tablet, a handset, or other type of mobile wireless computing device. The mobile device 300 can include one or more antennas 308 within housing 302 that are configured to communicate with a hotspot, base station (BS), an evolved NodeB (eNodeB), or other type of WLAN or WWAN access point. The mobile device 300 can be configured to communicate using multiple wireless communication standards, including standards selected from 3GPP LTE, WiMAX, High Speed Packet Access (HSPA), Bluetooth, and Wi-Fi standard definitions. The mobile device 300 can communicate using separate antennas for each wireless communication standard or shared antennas for multiple wireless communication standards. The mobile device 300 can communicate in a WLAN, a WPAN, and/or a WWAN.

FIG. 3 also shows a microphone 320 and one or more speakers 312 that can be used for audio input and output from the mobile device 300. A display screen 304 can be a liquid crystal display (LCD) screen, or other type of display screen such as an organic light emitting diode (OLED) display. The display screen 304 can be configured as a touch screen. The touch screen can use capacitive, resistive, or another type of touch screen technology. An application processor 314 and a graphics processor 318 can be coupled to internal memory 316 to provide processing and display capabilities. A non-volatile memory port 310 can also be used to provide data input/output options to a user. The non-volatile memory port 310 can also be used to expand the memory capabilities of the mobile device 300. A keyboard 306 can be integrated with the mobile device 300 or wirelessly connected to the mobile device 300 to provide additional user input. A virtual keyboard can also be provided using the touch screen. A camera 322 located on the front (display screen) side or the rear side of the mobile device 300 can also be integrated into the housing 302 of the mobile device 300.

FIG. 4 is a block diagram illustrating an example computer system machine 400 upon which any one or more of the methodologies herein discussed can be run. Computer system machine 400 can be embodied as the base station 102, the antennas 104, the user equipment 106, or any other computing platform described or referred to herein. In alternative embodiments, the machine operates as a standalone device or can be connected (e.g., networked) to other machines. In a networked deployment, the machine can operate in the capacity of either a server or a client machine in server-client network environments, or it can act as a peer machine in peer-to-peer (or distributed) network environments. The machine can be a personal computer (PC) that may or may not be portable (e.g., a notebook or a netbook), a tablet, a set-top box (STB), a gaming console, a Personal Digital Assistant (PDA), a mobile telephone or smartphone, a web appliance, a network router, switch or bridge, or any machine capable of executing instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

Example computer system machine 400 includes a processor 402 (e.g., a central processing unit (CPU), a graphics processing unit (GPU) or both), a main memory 404 and a static memory 406, which communicate with each other via an interconnect 408 (e.g., a link, a bus, etc.). The computer system machine 400 can further include a video display unit 410, an alphanumeric input device 412 (e.g., a keyboard), and a user interface (UI) navigation device 414 (e.g., a mouse). In one embodiment, the video display unit 410, input device 412 and UI navigation device 414 are a touch screen display. The computer system machine 400 can additionally include a storage device 416 (e.g., a drive unit), a signal generation device 418 (e.g., a speaker), an output controller 432, a power management controller 434, and a network interface device 420 (which can include or operably communicate with one or more antennas 430, transceivers, or other wireless communications hardware), and one or more sensors 428, such as a Global Positioning Sensor (GPS) sensor, compass, location sensor, accelerometer, or other sensor.

The storage device 416 includes a machine-readable medium 422 on which is stored one or more sets of data structures and instructions 424 (e.g., software) embodying or utilized by any one or more of the methodologies or functions described herein. The instructions 424 can also reside, completely or at least partially, within the main memory 404, static memory 406, and/or within the processor 402 during execution thereof by the computer system machine 400, with the main memory 404, static memory 406, and the processor 402 also constituting machine-readable media.

While the machine-readable medium 422 is illustrated in an example embodiment to be a single medium, the term “machine-readable medium” can include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more instructions 424. The term “machine-readable medium” shall also be taken to include any tangible medium that is capable of storing, encoding or carrying instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present disclosure or that is capable of storing, encoding or carrying data structures utilized by or associated with such instructions.

The instructions 424 can further be transmitted or received over a communications network 426 using a transmission medium via the network interface device 420 utilizing any one of a number of well-known transfer protocols (e.g., HTTP). The term “transmission medium” shall be taken to include any intangible medium that is capable of storing, encoding, or carrying instructions for execution by the machine, and includes digital or analog communications signals or other intangible medium to facilitate communication of such software.

It should be understood that the functional units or capabilities described in this specification can have been referred to or labeled as components or modules, in order to more particularly emphasize their implementation independence. For example, a component or module can be implemented as a hardware circuit comprising custom very-large-scale integration (VLSI) circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A component or module can also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like. Components or modules can also be implemented in software for execution by various types of processors. An identified component or module of executable code can, for instance, comprise one or more physical or logical blocks of computer instructions, which can, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified component or module need not be physically located together, but can comprise disparate instructions stored in different locations which, when joined logically together, comprise the component or module and achieve the stated purpose for the component or module.

Indeed, a component or module of executable code can be a single instruction, or many instructions, and can even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data can be identified and illustrated herein within components or modules, and can be embodied in any suitable form and organized within any suitable type of data structure. The operational data can be collected as a single data set, or can be distributed over different locations including over different storage devices, and can exist, at least partially, merely as electronic signals on a system or network. The components or modules can be passive or active, including agents operable to perform desired functions.

Additional examples of the presently described method, system, and device embodiments include the following, non-limiting configurations. Each of the following non-limiting examples can stand on its own, or can be combined in any permutation or combination with any one or more of the other examples provided below or throughout the present disclosure.

Example 1 includes the subject matter embodied by a method for selecting at least one parameter for downlink data transmission with a mobile user equipment, the method executable by a wireless communication base station having multiple antennas configured to communicate wirelessly with the mobile user equipment, the method including receiving an uplink probing signal from the mobile user equipment; determining, from the received uplink probing signal, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas; transmitting, from the multiple antennas, a plurality of downlink probing signals directionally toward corresponding angles-of-arrival in the plurality of angles-of-arrival, each downlink probing signal being a virtual antenna port with respect to the mobile user equipment; receiving, from the mobile user equipment, channel state information; and composing, in response to the received channel state information, at least one of a rank indicator (RI), precoding matrix indicator (PMI), or modulating and coding scheme (MCS) for downlink data transmission to the mobile user equipment.

In Example 2, the subject matter of Example 1 can optionally include wherein each downlink probing signal appears to originate from a different location from the point of view of the mobile user equipment.

In Example 3, the subject matter of one or any combination of Examples 1-2 can optionally include wherein determining, from the received uplink probing signal, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas includes employing an angle-of-arrival estimation algorithm.

In Example 4, the subject matter of Example 3 can optionally include wherein the angle-of-arrival estimation algorithm: projects the received uplink probing signal toward different spatial signatures spanning the angular space; determines peak values from the projection; and adopts the peak values as estimated angles-of-arrival.

In Example 5, the subject matter of one or any combination of Examples 1-4 can optionally include wherein the uplink probing signal and the downlink probing signals are at different frequencies.

In Example 6, the subject matter of Example 5 can optionally include wherein determining the downlink probing signal, in response to the uplink probing signal, includes: determining an uplink channel vector; multiplying the uplink channel vector by a diagonal matrix to form a product; and adopting the product to be the downlink channel vector.

In Example 7, the subject matter of Example 6 can optionally include wherein the diagonal matrix includes, at the Mth row and column, a complex exponential factor having an exponent that varies as (M−1) multiplied by the difference in frequency between the uplink probing signal and the downlink probing

In Example 8, the subject matter of one or any combination of Examples 6-7 can optionally further include forming a complex transpose of the product; and adopting the complex transpose as a defining vector for downlink virtual channel state information reference signal ports.

In Example 9, the subject matter of one or any combination of Examples 6-8 can optionally further include applying channel state information reference signals on top of the plurality of virtual antenna ports.

In Example 10, the subject matter of one or any combination of Examples 1-9 can optionally further include for virtual antenna ports numbering greater than four, downselecting the virtual antenna ports to a number fewer than four based on channel state information reference signal measurements.

Example 11 includes the subject matter embodied by a wireless communication base station having multiple antennas configured to communicate wirelessly with the mobile user equipment, the wireless communication base station including circuitry configured to: receive an uplink probing signal from the mobile user equipment; determine, from the received uplink probing signal, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas; transmit, from the multiple antennas, a plurality of downlink probing signals directionally toward corresponding angles-of-arrival in the plurality of angles-of-arrival, each downlink probing signal being a virtual antenna port with respect to the mobile user equipment; receive, from the mobile user equipment, channel state information; and compose, in response to the received channel state information, at least one of a rank indicator (RI), precoding matrix indicator (PMI), or modulating and coding scheme (MCS) for downlink data transmission to the mobile user equipment.

In Example 12, the subject matter of Example 11 can optionally include wherein each downlink probing signal appears to originate from a different location from the point of view of the mobile user equipment.

In Example 13, the subject matter of one or any combination of Examples 11-12 can optionally include wherein determining, from the received uplink probing signal, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas includes: employing an angle-of-arrival estimation algorithm.

In Example 14, the subject matter of Example 13 can optionally include wherein the angle-of-arrival estimation algorithm: projects the received uplink probing signal toward different spatial signatures spanning the angular space; determines peak values from the projection; and adopts the peak values as estimated angles-of-arrival.

In Example 15, the subject matter of one or any combination of Examples 11-14 wherein the uplink probing signal and the downlink probing signals are at different frequencies.

In Example 16, the subject matter of Example 15 can optionally include wherein determining the downlink probing signal, in response to the uplink probing signal, includes: determining an uplink channel vector; multiplying the uplink channel vector by a diagonal matrix to form a product; and adopting the product to be the downlink channel vector.

In Example 17, the subject matter of Example 16 can optionally include wherein the diagonal matrix includes, at the Mth row and column, a complex exponential factor having an exponent that varies as (M−1) multiplied by the difference in frequency between the uplink probing signal and the downlink probing signal.

In Example 18, the subject matter of one or any combination of Examples 16-17 can optionally include wherein the circuitry is further configured to: form a complex transpose of the product; and adopt the complex transpose as a defining vector for downlink virtual channel state information reference signal ports.

In Example 19, the subject matter of one or any combination of Examples 16-18 can optionally include wherein the circuitry is further configured to: apply channel state information reference signals on top of the plurality of virtual antenna ports.

Example 20 includes the subject matter embodied by a method for selecting at least one parameter for downlink data transmission with a mobile user equipment, the method performed by a wireless communication base station having multiple antennas configured to communicate wirelessly with the mobile user equipment, the method including: receiving an uplink probing signal from the mobile user equipment; employing an angle-of-arrival estimation algorithm; determining, from the angle-of-arrival estimation algorithm, a plurality of angles-of-arrival for a corresponding plurality of paths between the mobile user equipment and the multiple antennas; transmitting, from the multiple antennas, a plurality of downlink probing signals directionally toward corresponding angles-of-arrival in the plurality of angles-of-arrival, each downlink probing signal being a virtual antenna port with respect to the mobile user equipment, each downlink probing signal appearing to originate from a different location from the point of view of the mobile user equipment the uplink probing signal and the downlink probing signals having different frequencies; receiving, from the mobile user equipment, channel state information; and composing, in response to the received channel state information, at least one of a rank indicator (RI), precoding matrix indicator (PMI), or modulating and coding scheme (MCS) for downlink data transmission to the mobile user equipment.

The Abstract is provided to allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment. 

1. (canceled)
 2. A wireless device comprising: processing circuitry configured to: receive a first probing signal from a second wireless device, wherein the first probing signal is received via a plurality of spatial paths from the second wireless device to said wireless device; in response to receiving the first probing signal, determine a plurality of signal measurements associated with the plurality of spatial paths, wherein each of the signal measurements is associated with a corresponding one of the plurality of spatial paths; and transmit a plurality of second probing signals for transmission to the second wireless device, wherein each of the plurality of second probing signals is precoded for directional transmission to the second wireless device on a corresponding one of the plurality of spatial paths, based on the plurality of signal measurements.
 3. The wireless device of claim 2, wherein the processing circuitry is further configured to: transmit each of the plurality of second probing signals using a corresponding virtual antenna port.
 4. The wireless device of claim 3, wherein the processing circuitry is further configured to: configure each virtual antenna port based on a corresponding discrete Fourier transform (DFT) vector.
 5. The wireless device of claim 2, wherein the plurality of signal measurements comprise a plurality of angles-of-arrival corresponding to the plurality of spatial paths.
 6. The wireless device of claim 5, wherein the processing circuitry is further configured to: determine the plurality of angles of arrival in part by projecting the received first probing signal onto spatial signatures corresponding to a set of candidate directions.
 7. The wireless device of claim 2, further comprising: transceiver circuitry coupled to the processing circuitry; and a plurality of antennas coupled to the transceiver circuitry.
 8. The wireless device of claim 7, wherein each antenna of the plurality of antennas receives the first probing signal via the plurality of spatial paths.
 9. The wireless device of claim 2, wherein the processing circuitry is further configured to: receive channel state information (CSI) from the second wireless device, wherein the channel state information corresponds to a channel associated with at least one of the plurality of second probing signals; and in response to receiving the channel state information, encode a rank indicator (RI) and/or a precoding matrix indicator (PMI) for transmission to the second wireless device.
 10. The wireless device of claim 9, wherein the processing circuitry is further configured to: in response to said receiving the channel state information, select a modulating and coding scheme (MCS); and transmit data to the second wireless device using the selected MCS, wherein each of the plurality of second probing signals appears to originate from a different location from the point of view of the second wireless device.
 11. The wireless device of claim 2, wherein the processing circuitry is further configured to: project the received first probing signal toward different spatial signatures sampling the angular space; determine peak values from the projection; and adopt the peak values as estimated angles-of-arrival for the plurality of signal measurements.
 12. A wireless user equipment (UE) device comprising: processing circuitry configured to: transmit a first probing signal to a second wireless device; receiving a plurality of second probing signals from the second wireless device, wherein each of the second probing signals corresponds to a different virtual antenna port.
 13. The wireless UE device of claim 12, wherein the second probing signals are received in known time-frequency resources corresponding to channel state information reference signals (CSI-RSs).
 14. The wireless UE device of claim 12, wherein the processing circuitry is further configured to: perform antenna switching during said transmission of the first probing signals.
 15. The wireless UE device of claim 12, wherein the first probing signal is transmitted at a frequency different from one or more receive frequencies corresponding to the second probing signals.
 16. The wireless UE device of claim 12, wherein the processing circuitry is further configured to: receive an indication of a number of the second probing signals.
 17. The wireless UE device of claim 12, wherein the processing circuitry is further configured to: perform channel estimation based on one or more of the second probing signals to obtain channel state information (CSI); and transmitting the CSI to the second wireless device.
 18. The wireless UE device of claim 12, wherein the processing circuitry is further configured to: after transmitting the channel state information, receiving a rank indicator (RI) and/or a precoding matrix indicator (PMI) from the second wireless device.
 19. A method for operating a user equipment (UE) device, the method comprising: transmit a first probing signal to a second wireless device; and receiving a plurality of second probing signals from the second wireless device, wherein each of the second probing signals corresponds to a different virtual antenna port.
 20. The method of claim 19, wherein the first probing signal is transmitted via a plurality of spatial paths from the wireless UE device to the second wireless device.
 21. The method of claim 19, further comprising: receive each of the plurality of second probing signals using the corresponding virtual antenna port. 